1. Field of the Invention
The invention relates to radars, communications systems, and other electrical applications that employ microwave circuits in conjunction with high voltage DC blocks. High voltage DC blocks of this nature have applications in vacuum tubes and IMPATT devices, as well as ferro-electric or electrooptic phase shifters. They are also employed to protect bias tees and electrical devices that employ bias tees.
2. Description of the Prior Art
An inexpensive, compact, easily manufacturable high voltage DC block within devices employing microwave circuits is needed to allow the use of higher voltages in coupled line filters than previously possible. Conventional DC voltage blocks are effective up to approximately 200 volts. The DC voltage block of the instant invention is effective up to at least 4500 volts. DC voltage blocks in conventional microcircuits consist of coupled line filters as discussed in D. Lacombe and J. Cohen, "Octave-band microstrip DC blocks," IEEE Trans. Microwave Theory Tech., vol. MTT-20, pp. 555-556, Aug. 1972; B.A. Syrett, "A broad-band element for microstrip bias or tuning circuits,"IEEE Trans. Microwave Theory Tech., vol. MTT-28, pp. 925-927, Aug. 1980; S.R. Borgaonkar and S.N. Rao, "Analysis and design of D.C. blocks," Electronics Lett., vol. 17, pp. 101-103, Jan. 1981; B.J. Minnis, "Printed circuit coupled-line filters for bandwidths up to and greater than an octave," IEEE Trans. Microwave Theory Tech., vol. MTT-29, pp. 215-222, Mar. 1981. It has been recognized previously that the use of overlays superimposed upon coupled lines is a technique for ameliorating voltage breakdown. J. L. Klein and K. Chang, "Optimum dielectric overlay thickness for equal even- and odd-mode, phase velocities in coupled microstrip circuits," Electronics Lett., pp. 274-276, Mar. 1990. Standard coupled line filters perform well up to 200 voltages, but with higher voltages they are subject to voltage breakdown. A ground plane DC block was described in T.E. Koscica, "Wide-band ground-plane DC block and bias feed," IEEE Trans. Microwave Theory Tech., vol. MTT-38, pp. 805-806, June 1990. Thius DC block was further improved and described in T.E. Koscica, "High Voltage Microwave DL Block for Microstrip Ground Planes, " Electronics Lett., vol. 26, pp. 1287-1288 August 1990 which withstands voltages as high as 4 kilovolts. In the Koscica article, a DC block was described which was coated with silicone rubber to reduce voltage breakdown through the air. This ground plane DC block was limited by three factors. First, voltage breakdown is a function of the gap length across which voltage is applied. Any processing flaws along the gap length of ground plane DC block results in reduced voltage breakdown protection. Second, it is not amenable to modeling techniques, but is instead dependent upon empirical data. And third, silicone rubber contains acid which acts as an oxidizing agent to copper.